Leak testing of various components (or systems) which are required to be gas-tight or liquid-tight, such as heat exchanger coils, fuel tanks, pressure vessels, fuel or hydraulic lines, etc., is a common step in the manufacturing process of such components. It is known in the art of leak testing to detect various structural flaws in components, such as leaky joints, cracks, porosity, and the like, by pressurizing the component with a gas and detecting trace quantities of the gas leaking from such components.
One way to detect such leaking gas is to use a known "photo-acoustic" effect. The photo-acoustic effect, as is known, occurs when the gas absorbs a beam of light having a particular wavelength. When the gas absorbs such a beam, the absorbed optical energy heats the gas, thereby causing it to expand. As the heated gas expands, it produces pressure or acoustic waves (i.e., sound) which propagate from the point of heating. An acoustic sensor, such as a microphone, detects the acoustic waves and produces an electrical signal indicative of the acoustic waves.
The electrical signal may also represent acoustic waves produced from background noise, which is generated by two distinct sources. The first source of background noise is the component. When the component being tested absorbs the beam energy, the component produces background noise in the form of an acoustic wave. The second source of acoustic background noise is the surrounding environment such as machinery, passing vehicles, compressed air, etc. As discussed in pending U.S. patent application Ser. No. 08/835,043, now U.S. Pat. No. 5,834,632, a second beam can be used to generate a baseline acoustic signal indicative of the acoustic background noise. The second beam, having a wavelength, which the leaking gas does not absorb, scans the component. The acoustic sensor detects and delivers an electrical signal indicative of the background noise to a processor, which also receives an electrical signal indicative of the acoustic wave produced by the gas and the background noise. The processor then subtracts the baseline acoustic signal from the electrical signal generated by the first beam yielding a signal indicative of only the leaking gas.
The comparative acoustic signals are produced by contacting the same point on the component with alternating first and second beams. The sensitivity of the leak detection system is proportional to the rate at which the comparative acoustic signals are produced, which, in turn, is a function of the rate at which the first and second beams alternatively contact the point under test. The comparison rate of the electrical signals generated by the first and second beams, however, limits the sensitivity of the leak detection system since the comparison rate is restrained by the rate at which the alternating first and second beams are produced. Existing switching techniques used to produce alternating first and second beams utilize the mechanical movement of numerous components, such as multiple mirrors, beam combiners, etc., thereby demanding excess space and increased cost. Relying upon the mechanical movement of the above mentioned components to produce alternating beams, thereby limits the production of alternating first and second beams to about 100 HZ. The rate of mechanical switching does not ensure that the first and second beams contact the same point on the component. Specifically, it is possible that the component or device directing the first and second beams may move between the alternating cycle, thereby allowing one of the alternating beams to contact a different point on the component which decreases the accuracy of the leak detection system. Although existing switching techniques may be used to alternate the first and second beams, the existing switching techniques are limited in speed to a rate of mechanical switching, thereby decreasing the sensitivity and accuracy of the leak detection system.
Existing switching techniques also fail to produce acoustic signals truly indicative of (1) the background noise and (2) the acoustic wave produced by the gas and the background noise. Without such accurate acoustic signals, the sensitivity of the leak detection decreases.